1. Field of the Invention
The present invention relates to an anchoring element for supporting a joint mechanism of a reconstructed joint, a method of reconstructing an ankle or hip joint, and a reconstructed joint.
The preferred embodiments of the invention will be described in the following with reference to the reconstruction of ankle and hip joints. However, the invention is not limited thereto. The invention can be used for other similar joints, particularly load-carrying joints. The invention can also be used in reconstructions after amputation or other defects.
2. Description of the Related Art
For some considerable time, attempts have been made to replace damaged joints by artificial prostheses. These have been anchored in the bone tissue, either directly or via an intermediate adhesive layer, usually bone cement. Various materials have been used for the prostheses, such as stainless steel, chrome and cobalt alloys, titanium alloys, aluminum alloys, ceramic material, carbon fiber, and many other materials. Great inventiveness has been exhibited in the surfaces and surface properties proposed for obtaining the best possible anchoring stability.
The greatest problem in orthopedic prosthesis surgery is still how best to secure the components of the prostheses to the patient's bone tissue. Experiments entailing mechanical locking between implant and bone tissue (press-fit, see for instance EP 149 527), and the use of bone cement (polymethylmethacrylate) have not been sufficiently successful.
Conventionally, prostheses are secured within bones with cement. Great care is taken to remove the marrow from the exposed bone--the cavity is even flushed clean--before the prosthesis is inserted and then secured to the, bone with cement. However, cemented prostheses often loosen. This is particularly the case with prostheses cemented within load-carrying joints, such as ankle and hip joints. It has also been found that, if used before hardening, bone cement tends to seep out into the adjacent bone tissue. Initially, this offers a certain mechanical stability but with young, overweight or more active individuals, the joint between bone and cement is finally destroyed, resulting in micro-movement. A layer of connective tissue grows between prosthesis and bone, and the joint finally loosens.
The other method used today is the "cementless" method which aims at biologically securing the prosthesis, i.e., direct contact is desired between prosthesis and bone tissue with no intermediate layer of connective tissue or adhesive. To achieve satisfactory anchoring stability, various materials and surface structures for the prosthesis have been tested and a technique known as "press-fit" has been used to retain the prosthesis in position after insertion. However, recently published experiments (see Schimmel et Huiskes, "Primary fit of the Lord cementless total hip", Acta. Orthop. Scand. 1988: 59(6): 638-642) indicate that this method does not function in reality. It has been shown that extremely unfavorable threepoint loading with stress concentrations is obtained (EP 176 711).
The problem cannot be solved merely by selecting the "correct surface or material". There are a number of other factors of decisive significance.
Attempts have therefore been made in recent years to use titanium fixtures anchored in the marrow cavity of the bone to become osseo-integrated, as described by Hagert et al. "Metacarpophalangal Joint Replacement with Osseo-integrated Endoprostheses" in Scand. J. Plast. Reconstr. Surg. 20: pages 207-218, 1986.
It is known to permanently anchor oral and extra-oral prostheses in bone tissue. This osseo-integration technique for dentistry has been developed over the last 25 years by Professor Branemark and his colleagues, with excellent results in applying fixtures in the jawbone to hold teeth or arch attachments. However, the experiments performed by Hagert to apply this technique to the reconstruction of finger joints have not fulfilled expectations. The unacceptable results are evidently due to the entirely different conditions encountered when using this "dental technique" in the prosthetic reconstruction of fingerjoints. These problems are of course increased in the case of ankle and hip joint prostheses since totally different loads are placed on these joints.
Today, the main problem in orthopedic prosthesis surgery is still loosening of the bone anchoring unit. However, with a success rate for dental implants of more than 90% over a 20 year period, a number of other problems arise which, so far, have been unnecessary to take into account. One of the major problems is increased wear on the joint mechanism. A different type of prosthesis design from that used hitherto is required if the osseo-integration method is to be applied. To enable the joint mechanism to be replaced without disturbing the bone-anchorage, the prosthesis system must be divided into components where the joint mechanism element can be separated from the actual bone-anchoring element. Furthermore, if the two-stage method is used, it must be possible to connect the joint mechanism in the second stage if the patient, or at least the patient's reconstructed joint, is not to be kept immobilized. Two factors must therefore be taken into account: First, the joint mechanism is subject to wear and therefore must be replaceable. Second, to use the two stage method, the joint mechanism must be replaceable.